Editing Data dredging (section)

=== Systematic bias ===
{{main article|Bias}}
Bias is a systematic error in the analysis. For example, doctors directed [[HIV]] patients at high cardiovascular risk to a particular HIV treatment, [[abacavir]], and lower-risk patients to other drugs, preventing a simple assessment of abacavir compared to other treatments. An analysis that did not correct for this bias unfairly penalized abacavir, since its patients were more high-risk so more of them had heart attacks.<ref name="Deming" /> This problem can be very severe, for example, in the [[observational study]].<ref name="Deming" /><ref name="bmj02">
{{Cite journal
|author1=Davey Smith, G.|author1-link=George Davey Smith
|author2=Ebrahim, S.
|title = Data dredging, bias, or confounding
|journal = BMJ
|volume = 325
|year = 2002
|pmc = 1124898
|doi = 10.1136/bmj.325.7378.1437
|pmid=12493654
|issue=7378
|pages=1437–1438}}
</ref>

Missing factors, unmeasured [[confounders]], and loss to follow-up can also lead to bias.<ref name="Deming" /> By selecting papers with significant [[p-value|''p''-values]], negative studies are selected against, which is [[publication bias]]. This is also known as ''file drawer bias'', because less significant ''p''-value results are left in the file drawer and never published.